Scroll-type fluid displacement devices are well known. For example, U.S. Pat. No. 801,182 to Leon Creux, discloses a scroll device including two scroll members each having a circular end plate and a spiroidal or involute scroll element. These scroll elements have identical spiral geometry and are interfit at an angular and radial offset to create a plurality of line contacts between their spiral curved surfaces. Thus, the interfit scroll elements seal off and define at least one pair of fluid pockets. By orbiting one scroll element relative to the other, the line contacts are shifted along the spiral-curved surfaces, thereby changing the volume of the fluid pockets. This volume increases or decreases depending upon the direction of the scroll elements' relative orbital motion and, thus, the device may be used to compress or expand fluids.
U.S. Pat. No. 4,676,075 to Masao Shiibayashi discloses a scroll-type device wherein an oil injection system is used which includes injection ports whose size is greater than the thickness of the scroll wrap. This prevents the injection ports from being completely blocked by the wrap of the orbiting scroll. However, an injection port with a large diameter can create significant leakage of gas from compression chambers at higher pressure to those at lower pressure. Particularly, during start up when the pressure in the oil sump is lower than the pressure in the intermediate compression chamber where the oil injection port is located, the gas in the intermediate compression chamber will flow in the opposite direction, i.e. through the oil injection port to the oil sump. This blocks the oil flow from the oil sump to the compression chamber and results in overheating of the compressor.
In some devices it is impossible for oil injection ports to be larger than the thickness of the scroll wrap. For instance, the scroll geometry incorporated in the device disclosed in the U.S. Pat. No. 5,458,471 to Shimao Ni employs variable vane thickness (see FIGS. 2 and 3). The portion of the orbiting scroll wrap by which the oil injection ports on the fixed scroll member would be covered is so thick that the oil injection port size cannot practically be larger than the corresponding thickness of the orbiting scroll wrap without seriously hurting compressor performance.